Base gasket leak around the stud hole can cause the issue you're seeing.  

(A 1 psi per minute leak is far more than I'd expect from a porous casting but anything is possible.)

I've had a two 360's leak around the front motor mount bolt, On one, the bolt wasn't tight and tightening didn't stop the leak...  had to reseal the cases.  On the other I suspect from the looks that the bolt had been loose in the past.  It also needed case sealed. 

1970 doesn't have a compression release...  that can be a source of "hidden" leaks.

Removing the ignition to spray the crank seal with soapy water is worth the effort...  on the clutch side a "shortcut" can be to plug the trans vent hose and keep the pressure at 5 psi for a few minutes.  If pressure builds up (a gentle "whoosh" when the trans vent is unplugged), then I pull the clutch cover.  No "whoosh" doesn't always mean no leak...  so sometimes I still pull the cover if the leak can't be found anywhere else.    

Also, I use a thin coat of semi-hardening Yamabond (or Permatex Motoseal) on the intake gaskets and base gasket which, while technically not suggested by Yamaha, has always worked well for me.  Especially on reed valve models which almost always seep around the intake gaskets even when everything is brand new.  

Pressure & vacuum testing seems to case a lot of headaches. I happy to have never done one.
Talking hydro-lock though back in the early "70's i was fitting the cover on a Husky & oiling the screw holes with probably engine oil which caused exacly that, went CRACK--.
Only i heard it haha & it cracked along under the threaded hole.
Tightened up ok so said nothing, but didn't use oil after that although probably gave a squirt of crc or wd40 maybe.

Yeah, I just assumed that the tapped holes went all the way through and could be leaky. When I pulled the jug and saw they were blind holes that eliminated that possibility. So I put it back on with a new gasket and sealed it with Three Bond 1184. That was fun, that stuff drys fast. I didn't know how long it would take me to get the piston in, so I started it in the cylinder with the gasket below it, propped up the cylinder while I applied the 3 Bond to the gasket and seated the cylinder. Whew.
I put the head back on and let it set overnight. Said a prayer and applied pressure. Still leaking in the same spot. Gonna have another look at the head. I hope its not the leaking around the sleeve.

Mark, I have the seal exposed on the points side, because I put a cheap seal in there a while ago and I'm going to replace it with an oem seal after I solve this. It's not leaking there though.

I wanted to test if the leak was a head / head gasket problem or a leak around the cylinder liner, so I thought if I made a cork gasket with the same diameters as the copper gasket and installed that it would seal the head (we're only talking 5 psi.) Any leak would have to be the liner. Well this time I saw no leak in the head gasket area so I guess the liner is ok? Must be the head leaks with the copper gasket.. BUT I did see something that I missed before. There is a hole in the casting that goes into the cylinder stud channel on the left front. Bubbles there. I'll try to link some pictures. The only way there could be a leak there is if I screwed up when I installed the jug the other day, and it is leaking around the gasket into that channel. I didn't use an oem gasket at the base of the jug, but I have one now and I'll try it again with the oem probably without sealant at first. One picture shows the bubbles, the other the hole.
drive.google.com/file/d/1JFZg_kkvgQyBsS1...1L0/view?usp=sharing
drive.google.com/file/d/1DsfPnXoQNyUaQWf...p9Q/view?usp=sharing

I've had great success with using K&W copper coat to seal the head.  I put a thin coat on both sides of the gasket and a thin coat on the cylinder and head surfaces.  

P.S.  When using Three Bond products years ago for two stroke assembly, my research led me to the 1215 product, not the 1184 or 1104 products.  Having used all 3, they are similar but the 1215 is more "forgiving", stays more flexible, and is easier to clean up.  

Three Bond recommended 1215 to me as the best product they had for two stroke engine assembly (and is allegedly the same as the Yamabond used to seal cases.)

If I had to use Three Bond and couldn't get 1215, I'd use 1104 to coat the base and intake gaskets.  

Or like I probably mentioned, Permatex Motoseal works great and is available at chain auto parts stores or Amazon at decent prices. www.autozone.com/sealants-glues-adhesive...er-2-7-oz/477972_0_0

?? what's with the ground out studs??

I see bubbles? half way up the jug. YOU said base.

Thanks for the replies, guys. Mark I appreciate the recommendations on the 3 Bond gasket seal products. Very good to know. I'll give an update when I get back to it.